I went with 7 (NeoGeo 4 in a row on top), but discovered that I prefer to use my thumb on the first button on the second row for D rather than the 4th button on top.
The MAME defaults assume you have a 6 button layout starting with 1 on the upper left. Use that layout and almost all games will be preconfigured.
So, if planning on only running arcade emulators, I'd stick with 6. If planning on running Steam games or hooking it up to an xbox360, go with 8. I have hacked x360 fightpads and play steam games on my cab. I've been screwed over on a few games by not having that 8th button.